1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, in which a gate insulating film and a gate electrode layer are formed on a semiconductor substrate and thereafter, a trench is formed and an insulating film is buried in the trench.
2. Description of the Related Art
In semiconductor devices such as flash memories, an element isolation region for isolating adjacent element regions from each other has been narrowed with recent development in refinement of elements. For example, Japanese publication, JP-A-2004-111547, discloses a semiconductor device in which an element isolation region with a shallow trench isolation (STI) structure is formed in a surface layer of a semiconductor substrate to zone an element region.
According to a fabricating method disclosed in JP-A-2004-111547, a gate insulating film is formed on a semiconductor substrate, and a polycrystalline silicon film is formed on the gate insulating film. A silicon nitride film and a silicon oxide film are further deposited on the polycrystalline silicon film in turn. A resist is formed on the silicon oxide film into a desired pattern. The silicon oxide film is then processed with the patterned resist serving as a mask, and the resist is removed. Subsequently, a trench is formed in the silicon nitride film, polycrystalline silicon film, gate insulating film and an upper surface of the substrate by a reactive ion etching (RIE) method with the silicon oxide film serving as a mask.
However, the above-described method results in reaction products in sidewalls and bottom of the trench. In the above-noted Japanese publication, the reaction product is removed while bird beak tending to be produced in the gate insulating film is suppressed. Accordingly, vapor phase cleaning (VPC) is carried out at 80° C., instead of a conventional thermal oxidation process, so that the reaction product is removed. The above-noted Japanese publication discloses an acid treatment to be carried out subsequent to a dilute hydrofluoric acid treatment as the method of removing reaction products while bird beak is suppressed. However, the disclosed method has been found to be insufficient in removal of reaction products.